The present invention relates generally to PDC drill bits and more particularly to PDC drill bits that are capable of cutting a borehole that is larger than their own diameter. Still more particularly, the present invention relates to a bi-center PDC bit in which the under-reaming portion is positioned at the end of the bit so as to eliminate the torque that would otherwise result.
Bits that are capable of cutting a borehole that is larger than their own diameter have been known for some time. This capability was often accomplished by using a bit that was truncated across a portion of its circumference, so that the center point of the bit was laterally offset from its axis of rotation. U.S. Pat. No. 2,953,354 discloses a bit of this sort. However, early bits were all diamond bits, having hundreds of natural diamonds on their cutting surfaces. These diamonds, while durable, did not allow for aggressive cutting action. Thus, the amount of cutting performed on each revolution of the bit was relatively small. Because diamond bits do not aggressively engage the formation and because there is no way to control the force with which any given diamond engages the formation, it was not practical to stabilize diamond bits except by providing them with a balanced or inherently stable body shape. Thus, the amount of imbalance force that could be tolerated within a given bit was small. More recently, few experimental polycrystalline diamond compact (PDC) bits have attempted to incorporate an eccentricity. However, these eccentric bits were modifications from existing designs and therefore were not capable of handling the imbalance forces associated with under-reaming. Accordingly, the amount of imbalance force that these bits could tolerate was also small.
A bit having a body that is only slightly eccentric can be tolerated because the mass of the bit body is sufficient to keep it drilling about its intended rotational axis, i.e. drilling a hole slightly larger than its pass-through diameter. The amount of offset or eccentricity that could be used in a diamond bit was thus severely limited, as too much offset would cause the bit to precess, or "whirl" in the hole.
There are many instances in which it is desirable to increase the diameter of a borehole below a certain point in the hole by more than the amount possible with diamond or prior art eccentric PDC bits. The reason for increasing the borehole diameter may be a desire to increase the annular volume between the casing and the drill string to allow better cementing or gravel packing, a need to facilitate liner casing operations in sections where formation swelling occurs, or instances of slim hole high-angle re-entry drilling.
For these reasons, in many of the instances where it is desired to significantly increase the borehole diameter below a certain point, the under-reaming is typically accomplished with a special under-reaming tool. These tools typically comprise extendible reaming arms that are passed through the smaller, upper portion of the borehole in a retracted state, then extended and rotated so as to increase the diameter of a preexisting hole. Because of their relatively large number of moving parts, under-reamers are vulnerable to failure and breakage. In addition, under-reamers must be used in a pre-drilled hole, thus requiring the passage of two pieces of equipment through each length of borehole, namely the smaller diameter bit followed by the under-reamer.
To avoid the disadvantages associated with under-reamers, bi-center PDC bits were developed. Referring to FIG. 1, conventional bi-center bits 10 comprise a lower pilot bit section 12 and a longitudinally offset, radially extending reaming section 14. During drilling, the bit rotates about the axis 16 of the pilot section, causing the reaming section to cut a hole having a diameter equal to twice the greatest radius of the reaming section 14. Prior to drilling however, as the bi-center bit is passed through the upper portion of the hole, it shifts laterally, so that the rotational axis 16 is not centered within the hole. This shifting allows the bit to pass through a hole having a diameter 22 that is smaller than the diameter 24 of the hole that it will drill once it begins rotating. Thus, there are typically three diameters associated with bi-center bits. The first is the diameter 20 of the pilot bit section, which is the smallest diameter. The largest diameter is diameter 24, which is the diameter of the hole cut by the reaming section, and intermediate these is the pass-through diameter 22, which is the diameter of the smallest hole through which the reaming section will fit.
Referring now to FIG. 1A, a simplified profile 50 of a conventional-type bi-center bit is shown. Profile 50 corresponds generally to the prior art bit shown in FIG. 1, but is not intended to be a representation of the profile of the bit of FIG. 1. Profile 50 includes two curved sub-profiles 52, 54. Sub-profile 52 is the profile of the pilot bit and sub-profile 54 is the profile of the reaming section. Each sub-profile 52, 54 comprises a curve 52.sub.a, 54.sub.a, extending between a radially inner point and a radially outer point and terminating in a gage portion 52.sub.g, 54.sub.g. The inner point of sub-profile 52 lies on the axis of rotation of the bit. For purposes of discussion, at any given point on either sub-profile the angle between a line perpendicular to the sub-profile at that point and the axis of rotation is defined as a. It can be seen that for the profile shown in FIG. 1A, .alpha. increases from zero or negative at the inner point of sub-profile 52 to approximately 90.degree. at the gage portion 52.sub.g of sub-profile 52. At the intersection of sub-profiles 52 and 54, .alpha. decreases abruptly before increasing again to 90.degree. along curve 54.sub.a.
Still referring to FIG. 1A, when bi-center bits were first developed, the pilot sections 12 of those bits were stabilized in a stand-alone manner. While it was recognized that an imbalance force F.sub.R would result from rotation of the longitudinally spaced-apart asymmetric reaming section, it was believed that stand-alone stability in the pilot section would cause the reaming section 14 to maintain its intended rotational axis and thereby improve the operation of the whole bit. Over time, it was discovered that operation of the bit was actually improved by providing a large imbalance force F.sub.P on the pilot section. Following this development, bi-center bits have been designed so that the imbalance force resulting from rotation of the pilot section, F.sub.P, is maximized in a direction opposite to F.sub.R, in an effort to mitigate F.sub.R as much as possible.
However, because in a conventional bi-center bit the reaming section is longitudinally spaced apart from the pilot section, the two imbalance forces F.sub.P, F.sub.R are axially offset by a distance x, with the result that operation of the bit produces a turning moment on the bit around an axis normal to the rotational axis (an axis normal to the plane of the paper, as drawn). Because the forces are oppositely directed, the turning moment M is equal to the product of the difference between the magnitudes of the two imbalance forces and the distance x: EQU M=(F.sub.P -F.sub.R).multidot.x
For example, if F.sub.P is equal to 20% of the weight on bit (0.2 WOB), F.sub.R is equal to 0.3 WOB, and x is 10 inches, the magnitude of the turning moment M will equal the magnitude of the WOB, [10(0.1 WOB)]. If the difference between the magnitudes of the imbalance forces were greater, or if the distance x were greater than 10 inches, as it is likely to be in most conventional bi-center bits, the turning moment M would be even greater. This turning moment renders conventional bi-center bits more difficult to steer and tends to put undue torque on the drill string and other bottom hole assembly (BHA) components, which in turn increases the likelihood of failure and shortens the life of the BHA.
In addition the drilling center of conventional bi-center bits tends to fluctuate, with the result that the borehole does not have a consistent diameter. Finally, the fluid dynamics of bits such as that shown in FIG. 1 tend to be poor, with fluid flow being concentrated in only a few areas, which can reduce bit efficiency.
Hence, it is desired to provide a bi-center PDC bit that is capable of drilling a hole larger than its pass-through diameter and that provides superior directional control and steerability. It is further desired to provide a bi-center bit that has good fluid flow properties, exhibits no fluctuation of its drilling center, and reduces fluctuations in torque on the BHA, both around the drilling axis and perpendicular to it.